The following relates to medical imaging systems. It finds particular application to calibrating such systems. More particularly, it is directed towards calibrating the coincidence timing utilized in time-of-flight (TOF) measurements, such as those associated with Positron Emission Tomography (PET).
A conventional PET scanner has a plurality of radiation detector modules. The modules are arranged to facilitate positioning a subject such that the modules surround the subject. A radiopharmaceutical is administered to and/or ingested by the subject. The radiopharmaceutical produces radiation decay events in the subject that emit positrons. The positrons travel a relatively short distance before interacting with an electron in an electron-positron annihilation event that produces two oppositely directed gamma rays. The two oppositely directed gamma rays are detected by two different radiation detector modules as two substantially simultaneous radiation detection events that define a line of response (LOR) therebetween.
Each radiation detector module includes circuitry that facilitates determining a spatial location at which each event is received and an energy of each event, as well as other information. For example, each radiation detector module often includes one or more scintillators that produce a proportional burst or scintillation of light responsive to each gamma ray detection. However, there are time variations amongst scintillation crystals as to how quickly the radiation is converted into light, which introduces various time delays into the light signals. Further time delays are introduced due to the conversion of the light into electrical signals. Such time delays typically vary from conversion device to conversion device. With conventional PET scanners, photomultiplier tubes are commonly used to convert the light into electrical signals. The time delays associated with each tube can vary greatly from tube to tube. Additional time delays are introduced as the signals propagate through the amplifiers, reactive circuit components, along the wires, etc. to the point at which each event is digitally time-stamped. Conventionally, each channel is calibrated to compensate for these delays through time delay circuitry.
Technological advances in the medical imaging arts have led to PET scanners with a temporal resolution of about twenty-five picoseconds today and better resolution is expected in the future. With this resolution, time-delay circuits need to be accurately calibrated. Conventional time-delay calibration techniques tend to be complicated and use a large number of data channels, typically tens of thousands of channels. For example, with one technique, time-of-flight scanners are temporally calibrated by rotating a line source around a circular path and measuring the relative detection times for the 180 degrees coincident gamma rays. This calibration procedure typically includes expensive and complex equipment. In addition, if a stationary source is used, it typically is difficult to determine whether the detector at one end of the line of response is slow or the detector at the other end is fast, and the like. Further, processing techniques that minimize time differences tend to find local minimum rather than the absolute minimum, reducing calibration accuracy.
Thus, there is a need for improved calibration techniques that facilitate overcoming these deficiencies.